1. Field of the Invention
The present invention relates to a photoresist for forming a fine pattern used in manufacturing a large scale integrated circuit (LSI) and so on.
2. Description of the Background Art
FIGS. 3A and 3B are views schematically showing the conventional photolithography technique. Referring to FIG. 3A, a photoresist film 2 is formed by applying a photoresist material onto a substrate 1 such as silicon wafer. A photomask 3 is located between substrate 1 and a light source. A reduction optical system 4 for focusing light from the light source onto photoresist film 2 is provided between photomask 3 and photoresist film 2.
After forming photoresist film 2 on substrate 1 as shown in FIG. 3A, photoresist film 2 is irradiated with the light 5 from the light source through photomask 3 and reduction optical system 4. A predetermined portion of photoresist film 2 is exposed by this irradiation. Thereafter, photoresist film 2 of a reduced pattern according to the pattern of photomask 3 can be formed as shown in FIG. 3B by removing a portion sensitized in a developing process.
Almost parallel rays as shown in FIG. 4A are preferred as the light 5 of the light source used in an exposure process as stated above, in which a spread of diffraction of incident light is small and a beam width of the light does not substantially change in the film thickness of photoresist film 2. In such a case, as far as the light 5 reaches substrate 1, the film thickness of photoresist film 2 does not significantly affect the resolution.
In the existing photolithography process for manufacturing a large scale integrated circuit and so on, however, as reduction optical system 4 is generally used, parallel rays as shown in FIG. 4A cannot be formed. When the light 5 was focused on the surface of or in the vicinity of photoresist film 2 in order to reproduce a mask pattern on the surface of photoresist film 2, the spread of diffraction of incident light was wide as shown in FIG. 3A and the light beam width significantly changed with respect to the film thickness of photoresist film 2.
Such a resolution limitation of the photoresist film can be described as follows.
That is, if the light 5 includes totally parallel rays, passing through photoresist film 2 in parallel, the size of a collected light spot when the parallel rays are collected with an optical system is given as a radius r of Airy Disc and expressed by the following equation, which limits the resolution of the photoresist film: EQU r=(0.61.times..lambda.)/NA.
In this case, .lambda. represents the wavelength of the light source and, in practice, g rays or i rays of a Hg lamp are used for it. A method is now being considered in which X rays (&lt;100 nm) and so on obtained from an excimer laser (KrF; 248 nm, ArF;196 nm) or an SR are used as a light source in order to form a finer pattern. NA is an aperture of the optical system representing a ratio of an effective diameter to a focal length of an optical lens. The larger the aperture NA is, the smaller the diameter of the collected light spot becomes, so that a higher resolution can be expected. However, if the aperture NA becomes larger, the spread of light becomes wide in front of and behind the focal point, causing the depth of focal point to be smaller, so that various problems are caused for sensitizing a thick photoresist film. A maximum of 0.6, normally in the order of 0.45 is taken as a value of the aperture NA when it is used for projection onto a photoresist film in manufacturing an LSI and so on.
A description will now be made of a practical example. A film thickness in the order of 1 .mu.m is required for a typical monolayer photoresist film 2 while it is desired that an excimer laser (for example, KrF; 248 nm) expected as a light source for hyperfine treating should realize a resolution of line and space of 0.35 .mu.m to 0.25 .mu.m. According to the equation above, the aperture NA of the optical system in this case needs to be 0.44-0.61. In this case, the light beam for photoresist film 2 having a thickness in the order of 1 .mu.m is as shown in FIGS. 3A and 3B and if the focus is placed at the surface of photoresist film 2, the light beam spreads to 0.44-0.61 .mu.m at the boundary with the substrate. Accordingly, it is not possible to obtain a shape of pattern for line and space of a desired resolution. Then, if the aperture NA of the optical system is 0.4, the diameter of the light beam hardly spreads even at the boundary of photoresist film 2 with substrate 1, allowing a state of almost parallel rays as shown in FIGS. 4A and 4B. As a result, though a desired form of pattern can be obtained, the diameter of the collected light spot becomes large, i.e., 0.38 .mu.m, inevitably decreasing the resolution. It is reported in K. J. Polasko et al., IEEE, Electro. Dev. Lett. EDL-5, 1, p.24 (1984) that the sensitivity is increased when a photoresist is exposed by direct exposure using an excimer laser (KrF; 249 nm) and then the pulse power of the laser is increased.
As stated above, it can be seen that there is a trade-off relationship between a spot diameter of diffraction limitation and a depth of focal point with the wavelength of the light source being constant. That is, in patterning a thick photoresist film 2 by exposure, if an optical system is employed having an aperture ratio NA which is just enough for ensuring such a depth of focal point that the light 5 includes rays parallel to each other to some extent within photoresist film 2, the resolution must be sacrificed. If an optical system of a high aperture ratio NA is employed, the light 5 spreads inside photoresist film 2, so that a problem such as side etching is caused at the time of development, making it difficult to obtain a preferable pattern. At the moment, though a shorter wavelength of a light source and a higher aperture ratio of an optical system are adopted in order to increase integration density of very large scale integrated circuits, a thickness of photoresist film 2 can hardly be changed from that of the conventional ones due to the need for ensuring dry etching resistant characteristic and so on, so that it is still difficult to obtain a patterning film of a photoresist having a high aspect ratio and a high resolution.
Methods in which a photoresist film is used in a structure of combination of two or more layers have been developed for obtaining a photoresist film 2 of a required thickness by a post-treatment of lithography while raising such a resolution close to the diffraction limitation of the light. One of them is a method using a double-layered structure, in which a very thin photoresist film is formed on a thick film having no dry etching resistant characteristic. In this case, it is possible to collect light with an optical system of a high NA without considering a spread of diffraction of the directed light since the photoresist film is thin. Thereafter, a resist pattern form having a required thickness can be obtained with a high resolution by etching, using the thin photoresist layer as a mask in accordance with a method such as dry etching or the like.
Another method is CEL (Contrast Enhanced Lithography) in which a contrast enhancement layer is applied onto photoresist film 2. Since such a contrast enhancement layer strongly absorbs light used for exposure and discolors to pass the light, the contrast can be increased if the photoresist film is sensitized through the CEL layer.
Synthesizing of a material used for CEL is disclosed in Shou-ichi Uchino et al., J. Photopolymer Sci. Tech. 2, 1, p. 59-65 (1989) in which a resolution of a resist can be enhanced by improving the contrast of an optical image reduced by a reduction projection exposure device.
However, formation of such a photoresist film having the double-layered structure causes a problem of an increase in the number of manufacturing processes, which is significantly disadvantageous in manufacturing large scale integrated circuits of increased integration density.
Another one is a phase shifting method for forming a photoresist film having a required thickness with a high resolution. Since this method is intended for enhancement of the contrast by inversion of light phase for forming adjacent patterns with a phase shifting layer provided for a mask, manufacturing of the mask becomes complicated and it can only be employed for forming a pattern having the line and space periodically repeated.